The invention relates to electrolytic cells for electrowinning metals from a fused salt bath, especially aluminium from a fused cryolite-alumina bath comprising at least one anode immersed in said bath above a cathode disposed at the bottom of the cell. In conventional Hall-Heroult electrolytic cells for aluminium electrowinning, a molten aluminium pool of about 15 cm height or more is, for a variety of reasons, maintained at the bottom of the cell to provide a continuous surface for passage of the cathode current.
Movement of the molten aluminium due to strong magnetohydrodynamic and other effects leads to a variable surface of the aluminium pool and thereby imposes a minimum anode-cathode distance of about 4-6 cm.
It has been proposed to equip metal electrowinning cells with different types of cathode structures mounted on the cell bottom in order to allow the molten metal to be continuously drained off so that the anode-cathode distance may be reduced.
Thus, for example, U.S. Pat. No. 4,071,420 relates to a method of metal electrowinning, which comprises providing at least one hollow body which protrudes out of the molten metal pad, is open at its end closest to the anode surface, and is sealed at its end in the pad. The molten metal is thus caused to overflow at a fixed level from the open end of said hollow body.
U.S. Pat. Nos. 3,400,061 and 4,093,524 moreover relate to cells for aluminium electrowinning, which comprise an inclined cathode surface for draining off the molten aluminium except for a thin layer of molten metal wetting the cathode surface. However, the fabrication, precise positioning and fixation of such cathodic structures are both complicated and expensive, especially in the case of retrofitting existing electrolytic cells with such cathodes.
Thus, although a reduction of the anode-cathode distance would evidently be desirable for achieving significant energy savings, and in spite of the fact that considerable efforts have been devoted to developing wettable cathodes for this purpose, the technical difficulties of retrofitting existing cells or equipping redesigned cells with the cathodes proposed hitherto have been a major obstacle to achieving this purpose.